I'm not sure specifically as I haven't read this particular fanfic. But generally, in MLP fanfics, the plot could involve the ponies facing some new challenges, perhaps a new villain in Equestria, or it might be about a pony's journey to discover something extraordinary.
I don't know the main characters in this fanfic as I haven't read it. But in MLP, common main characters could be Twilight Sparkle, Pinkie Pie, Rainbow Dash, etc. Maybe this fanfic has a new or less - known character as the main focus along with some of the regular ones.
Perhaps the story in the 'drunk mlp fanfic' is centered around a group of ponies who accidentally consume some enchanted cider that makes them drunk. They then start seeing strange visions and go on a wacky journey to find a cure, all while their normal pony friends are trying to figure out what's wrong with them.
Well, it could be about a soldier who finds himself in Equestria by some strange means. He might have to adapt to the new environment which is full of ponies with different personalities and abilities. The fanfic could explore his interactions with Twilight Sparkle and her friends, perhaps he helps them in a time of crisis or they help him understand the magic of Equestria better.
Well, in a 'mlp through the mirror fanfic', it could start with a pony accidentally stumbling upon a magic mirror. This mirror might be enchanted in a way that it acts as a portal. Once through the mirror, the ponies could find a world where the rules are different. For example, the magic they know might work in strange ways. They may meet versions of themselves or other ponies that are quite different from what they're used to. It could be a story about self - discovery and adventure as they navigate this new and unfamiliar place.
Well, it could be about Starlight feeling wronged by the ponies in Ponyville. Maybe she decides to take revenge because she was excluded from some important events. So she starts using her magic to create small troubles for the ponies, like making their crops grow slowly or causing minor chaos during their festivals.
The plot might center around a portal that opens between the two worlds. The Hyrule Warriors characters, such as Link or Zelda, are transported to Equestria. There, they discover that the chaos in their own world is somehow related to a disturbance in the harmony of Equestria. So, they join forces with the ponies, like Twilight Sparkle and her friends. Along the way, they face challenges that require both the brute strength and combat expertise of the Hyrule Warriors and the magical abilities and friendship power of the ponies.
Perhaps in the fanfic, Luna has a nightmare about her past as Nightmare Moon. She wakes up crying because she fears that she could lose control again and turn back into that evil form. This would show her internal struggle with her own identity and the fear of repeating her past mistakes.
One possible plot could be that a pony has a really bad toothache and has to visit the dentist pony in Ponyville. The dentist pony discovers that there's a magical element causing the toothache and has to find a way to fix it without harming the patient. It could involve a journey to find a special herb or spell.
One possible plot could be that the ponies accidentally end up in the Attack on Titan world through some sort of magical portal. They then have to figure out how to survive in this harsh environment. They could use their unique pony abilities, like Fluttershy's connection with animals, to try and understand the Titans better. Maybe they could even find a way to communicate with the Titans and find out if there's a way to stop the attacks without all the bloodshed.
A possible plot could be that a human soldier gets lost in the mlp world. He has to adapt to the new environment which is very different from his own. He could use his combat skills to protect the ponies from a common enemy, like a dragon. As he stays longer, he starts to understand the pony culture and values, and in turn, the ponies learn about human courage and tenacity.